MEMS gyroscopes use the Coriolis effect to measure angular velocity. In a vibrating MEMS gyroscope, an inertial mass is driven into oscillating movement by an actuating drive force. This oscillation will be called “drive oscillation” in this disclosure. The drive oscillation can be either linear or rotational, but this disclosure focuses exclusively on applications where it is rotational. FIG. 1 illustrates schematically an inertial mass 111 driven in rotational oscillation about the z-axis. The drive oscillation is indicated with a solid black arrow. The actuating drive force can be generated, for example, with an electrostatic, magnetic or piezoelectric drive transducer. This disclosure focuses exclusively on applications the actuating force is generated piezoelectrically.
When a gyroscope containing an inertial mass in drive oscillation undergoes an angular rotation rate Ω about a secondary axis (not parallel to the primary axis), the inertial mass is affected by the Coriolis-effect which is accompanied by a Coriolis force and torque. The Coriolis force and torque are determined by the magnitude and direction of the angular rotation rate vector and the inertial mass velocity and angular velocity vectors. An inertial mass in linear drive oscillation will undergo an oscillating Coriolis force. An inertial mass in rotational drive oscillation will undergo an oscillating Coriolis torque. This force or torque oscillates the inertial mass about a secondary axis perpendicular to the primary axis. Oscillation along or about the secondary axis will be called “sense oscillation” in this disclosure.
In FIG. 1 an angular rotation rate Ω about the x-axis is indicated with a white arrow, and the resulting sense oscillation about the y-axis is indicated with a grey arrow. To measure the angular rotation rate Ω, the sense oscillation may be measured through a capacitive, piezoelectric or piezoresistive transducer. The resulting electrical signal may be called a sense signal. This disclosure focuses exclusively on applications where the sense oscillation is measured with a piezoelectric transducer.
Gyroscopes with a single, piezoelectrically actuated inertial mass in rotational oscillation are susceptible to disturbances arising from external rotary vibrations. They may also suffer from acoustic losses due to mechanical coupling from the inertial mass and the suspenders to the fixed base, because even a fixed base has a finite mass and may be mobile to a degree. These problems may be circumvented with gyroscopes which include two inertial masses. The two inertial masses may be mechanically coupled to each other to oscillate synchronously.
The two inertial masses can be driven into anti-phase oscillation. In this disclosure, “synchronous anti-phase oscillation” means oscillation where, at any given time during the oscillation cycle, the first mass rotates clockwise about a first axis when the second mass rotates at equal angular velocity counter-clockwise about a second axis which is parallel with the first axis. When the first mass turns from clockwise rotation to counter-clockwise rotation about the first axis, the second turns from counter-clockwise to clockwise rotation about the second axis. In anti-phase oscillation, the torques exerted by the two inertial masses on the fixed base will be equal but opposite, and will cancel each other so that no rotational oscillation will be coupled to the fixed base even if it has a finite mass or is not totally immobile. The effect of external rotational vibrations on each inertial mass will also be equal, and by reading the sense transducers in a differential manner this effect can be cancelled in the sense signal.
FIG. 2a illustrates schematically a gyroscope with two inertial masses 211 and 212 oscillating in two different directions about the z-axis perpendicular to the xy-plane. The arrows are reversed in the second oscillation phase where the masses rotate in the opposite direction. The xy-plane defines the initial rest position of the inertial masses. The xy-plane will be called the device plane in this disclosure. Oscillation about the z-axis which is perpendicular to this plane will be called in-plane oscillation, or oscillation in the device plane, in this disclosure. FIG. 2b illustrates schematically a gyroscope where the same inertial masses 211 and 212 oscillate in two different directions about the y-axis, which lies in the device plane. The degree of rotation has been greatly exaggerated. As they oscillate about the y-axis, the masses 211 and 212 rotate out of the device plane. This oscillation mode will be called out-of-plane oscillation, or oscillation out of the device plane, in this disclosure.
In both FIGS. 2a and 2b, the two inertial masses are coupled to each other by a transversal synchronization spring 26. With a suitably constructed synchronization spring, driving only one of the inertial masses 211 and 212 into drive oscillation with a drive transducer (not illustrated) is sufficient to set the other inertial mass into anti-phase drive oscillation with the same frequency. However, it is also possible to drive both inertial masses 211 and 212 into drive oscillation with two separate drive transducers. The sense oscillation, which will occur in both inertial masses, can be read through a sense transducer connected to either one of the inertial masses 211 and 212, or through two or more sense transducers, each one connected to either mass.
As illustrated in FIGS. 2a and 2b, inertial masses in piezoelectrically driven MEMS gyroscopes may have an oblong shape. They have a longitudinal length in the x-direction which exceeds their transversal width in the y-direction.
The terms “longitudinal” and “transversal” will be used throughout this disclosure to refer to the illustrated x- and y-directions, respectively. The longitudinal measure of an object may be referred to as a “length” and the transversal measure of an object will may be referred to as a “width”. However, since the synchronization spring 26 is oriented in the transversal direction (y-direction), its transversal measure may be referred to as its “length”, and its longitudinal measure may be referred to as a “width”. The term “vertical” will be used to refer to the z-direction, and the corresponding measure will be referred to as a “thickness”.
One way to build a piezoelectrically driven or sensed inertial mass is to shape the inertial mass so that it at least partly surrounds the anchor point or anchor points from which it is suspended. In other words, the inertial mass may be shaped like an open or closed frame and a suspension structure may be constructed between a centrally located anchor point and fixing points on the inner edge of the inertial mass.
Document WO2011136972 discloses a piezoelectric gyroscope where piezoelectric transducers have been placed on suspenders which suspend an inertial mass from a central anchor point.
Certain technical problems are frequently encountered in piezoelectrically driven and sensed gyroscopes. One is that the transducers which drive the oscillation must be sufficiently large to generate enough actuation force, and the transducers which sense oscillation must be sufficiently large to produce a signal with a high signal-to-noise ratio. Furthermore, the drive oscillation movement may not be perfectly orthogonal to sense oscillation movement, which may introduce drive motion error components into the sense signal. This can be serious problem because the amplitude of the drive oscillation is usually much larger than the amplitude of the sense oscillation. A further problem is that the bending mode of the piezoelectric transducers may not exhibit uniform curvature along the entire length of the transducer, which can reduce both the drive force and the sense signal.
These problems have hindered the development of piezoelectric rotational gyroscopes in comparison to electrostatically driven and sensed gyroscopes, even though electrostatic gyroscopes require high bias voltages, consume more surface area and produce a capacitive output signal which is inversely proportional to the operating frequency, making high frequency operation unpractical despite its advantageousness due to smaller sensitivity to external vibrations.